1. Field of the Invention
The present invention relates to a high-rigidity ethylene-propylene copolymer and a process for producing the same. More particularly it relates to an ethylenepropylene-copolymer obtained by employing a specified Ziegler-Natta system catalyst and a three-stage block copolymerization process, and having a high-rigidity, almost no whitening due to impact or folding and a superior high impact strength and heat resistance, and a process for producing the same.
2. Description of the Prior Art
Crystalline polypropylene (hereinafter abbreviated to polypropylene) obtained by polymerizing propylene has superior physical properties such as rigidity, heat resistance, etc., but on the other hand, a problem that it has a low impact strength, particularly that at low temperatures has been raised; thus the range of its practical uses has been limited. In order to overcome this drawback, a number of processes for block-copolymerizing propylene with ethylene or another .alpha.-olefin have been proposed. According to these copolymerization processes, it is possible to obtain a propylene-.alpha.-olefin copolymer having a superior low-temperature, high-impact strength without so much damaging the superior characteristics of polypropylene such as high-rigidity, heat resistance, etc. On the other hand, a drawback as described below occurred to such a copolymer. Namely, when molded products are produced from such a copolymer, or the molded products are transported or used, an impact or a folding force loaded on the molded products causes the loaded part of the products to readily whiten. The molded products which whitened like this, naturally lose their commodity value at all. A number of processes for overcoming such a drawback of propylene-.alpha.-olefin block copolymers (hereinafter abbreviated to block copolymers) have been proposed. For example, .circle.1 Japanese patent application laid-open Nos. Sho 55-58245/1980, Sho 55-104333/1980, Sho 56-72042/1981 and Sho 57-137341/1982 are directed to a process of blending polyethylene with block copolymers. Such a process has a whitening-improving effectiveness, which, however, is still insufficient, and commercially a problem of uniform blending has been raised and also the blending cost cannot be neglected. Next, .circle.2 Japanese patent publication Nos. Sho 47-26190/1972 and Sho 49-24593/1974 and Japanese patent application laid-open No. Sho 58-15548/1983 are directed to a multi-stage polymerization wherein propylene homopolymer is prepared at the first stage and successively ethylene is copolymerized with propylene at multi-stages. However, as to the block copolymers obtained in these cases, while rigidity reduction can be prevented, the whitening-improving effectiveness is insufficient. Next, .circle.3 Japanese patent application laid-open No. Sho 54-40895/1979 is directed to a process wherein hydrogen concentration is raised at the stage where an ethylene-propylene copolymer part is prepared, in the block copolymer production, to reduce the molecular weight corresponding to the part. In this case, however, the resulting block copolymer is insufficient in improving the low-temperature, high-impact strength. Next, .circle.4 Japanese patent application laid-open Nos. Sho 54-13963/1979, Sho 55-16048/1980, Sho 56-55416/1981, Sho 57-34112/1982 and Sho 57-67611/1982 are directed to an ethylenepropylene multi-stage polymerization process wherein a small amount of ethylene is fed in the production process of a propylene polymer part at the first stage and polymerization for an ethylene-propylene copolymer part is carried out successively at multi-stages i.e. at the second stage et seq. However, the resulting block copolymer is notably reduced in the rigidity and heat resistance which are specific features intrinsic of polypropylene; hence such a process cannot also be regarded as a desirable process.
On the other hand, a number of processes for improving the rigidity of block copolymer mainly by improving the catalyst used for the processes, have been proposed. Namely .circle.5 Japanese patent publication Nos. Sho 47-8207/1972, Sho 49-13231/1974 and Sho 49-13514/1974 are directed to an improved process wherein a third component is added to a catalyst. Further, .circle.6 Japanese patent application laid-open No. Sho 55-764/1980, Sho 54-152095/1979 and Sho 53-29390/1978 and Japanese patent publication No. Sho 55-8011/1980 are directed to an improved process of using a specified catalyst. However, the above proposed processes .circle.5 and .circle.6 are directed to a technique for making the reduced extent of the rigidity of the resulting block copolymers as small as possible, as compared with polypropylene (homopolymer), that is, so to speak, a technique for relieving their drawbacks, and the processes have not yet attained rigidity values same as or higher than those of the homopolymer.
The object of the present invention is to provide an ethylene-propylene copolymer having a high-rigidity and whitening resistance by which the problems of the prior art as raised in the above processes .circle.1 .about. .circle.6 have been solved according to the process described below. In order to attain the object, (i) we have specified the kind of stereoregular catalysts used, and (ii) have limited the polymerization conditions at the time of polymerizing propylene and ethylene at three stages.
As a result, all the above problems have been solved, and we have found a high-rigidity ethylene-propylene copolymer which has a superior heat resistance, rigidity and low-temperature, high-impact strength, causes no whitening phenomenon when stresses such as impact, folding, etc. are applied and results in a superior appearance when molded.